This invention relates to the field of extractive pyrometallurgy and, more particularly, to an improved method for recovery of lead from lead sulfide ores by reduction with metallic iron.
Conventionally, lead is extracted from lead sulfide (galena) ores by roasting an ore concentrate to form a lead oxide and reducing the lead oxide to molten lead in a blast furnace. The lead sulfide ore is initially concentrated by a flotation process to form a green concentrate, and the green concentrate is charged to a continuous sintering and roasting operation in which it is conveyed along a moving grate and air is passed through the grate to oxidize the PbS. To provide proper support for newly charged green concentrate and adequate porosity of the roasting mass, approximately 70% of the calcined (burned) material leaving the roasting zone must be recycled through the sintering and roasting operation where it serves as a bed on which the green concentrate is charged. The other thirty percent of the calcined product is charged forward to the blast furnace.
The calcine produced by roasting is predominantly lead oxide but also contains appreciable proportions of lead sulfide, lead sulfate and elemental lead. Together with a limestone flux and metallurgical coke, this material is charged to a blast furnace, where the lead oxide is reduced to elemental lead by reaction with carbon monoxide formed from the partial combustion of the coke. The effluent from the lower end of the blast furnace includes molten elemental lead, a slag, and a matte or dross which may be treated for recovery of copper and nickel.
Although reasonably economical and effective for recovery of lead from galena, operation of the conventional roasting process causes severe air pollution problems in the form of sulfur dioxide emissions. In the roasting operation, sulfur dioxide is formed as a by-product of the oxidation of lead sulfide: EQU 2PbS + 3O.sub.2 .fwdarw. 2PbO + 2SO.sub.2
further amounts of SO.sub.2 are generated by the oxidation of other sulfides contained in the concentrate, e.g., Cu, Fe and Zn sulfides. Because at least one mole of sulfur dioxide is formed for each pound atom of lead produced, tonnage volumes of sulfur dioxide are continuously generated in roasting galena concentrates. A portion of this sulfur dioxide can be converted to sulfuric acid in an on-site contact acid plant. Because of the progressive depletion of sulfur from the sinter/roasting bed as it passes to the discharge end of the roasting zone, however, the gas emanating from the downstream end of the roasting zone is too lean for practical conversion to sulfuric acid. Generally, no more than about 60% of the sulfur dioxide generated by roasting can be economically converted to sulfuric acid, and the remainder must be collected by lime scrubbing or other expensive pollution abatement procedures if massive emissions of sulfur dioxide to the atmosphere are to be avoided.
Because the strength of gas passing off the roasting bed not only varies with the position along the length of the bed but also varies with time due to the impossibility of precise control of the roasting operation, operating problems can be encountered with a by-product acid plant. Such problems add to the cost of converting the SO.sub.2 to sulfuric acid and raise the risk of further emissions of unconverted SO.sub.2 from the acid plant. In such circumstances, the overall SO.sub.2 recovery can be significantly less than 60%. When downtime is experienced in the acid plant, moreover, either the roasting operation must be shut down or 100% of the SO.sub.2 generated is released to the atmosphere.
As a result of increasing regulatory restrictions on allowable sulfur dioxide emissions, and the consequent need to install and operate ever more elaborate emission control systems, the cost of operating the roasting unit in a conventional lead extraction plant is expected to increase sharply. There is, thus, a need and an opportunity for commercial implementation of processes by which lead may be efficiently recovered from galena ores without significant generation of sulfur dioxide. One process which avoids SO.sub.2 generation, and might, therefore, be considered for commercial use, is the so-called salt process, described in Betts U.S. Pat. No. 821,330, in which reduction takes place according to the reactions: EQU Na.sub.2 CO.sub.3 + 2C + PbS .fwdarw. Pb + Na.sub.2 S + 3CO EQU fe.sub.2 O.sub.3 + 3C + 2PbS .fwdarw. 2Pb + 2FeS + 3CO
pendar U.S. Pat. No. 2,834,669 describes a modification of this process in which NaCl is substituted in part for Na.sub.2 CO.sub.3 to improve process economics. Since the salt process produces a matte containing Na.sub.2 S it presents severe disposal problems and may raise severe corrosion problems as well.
A hydrogen reduction process has also been proposed for lead recovery but this process not only requires relatively large amounts of hydrogen but generates at least a mole of hydrogen sulfide for each pound atom of lead that is produced. In operation of this process, it is necessary to separate the hydrogen sulfide by-product from relatively large volumes of unreacted hydrogen, and to make provision for disposal of the hdyrogen sulfide so separated.
In the 19th century, a process was carried out in Germany in which lead was recovered from lead sulfide ore by direct reduction with metallic iron. This process has been known as the "precipitation" process. As practiced commercially, the precipitation process did not afford high recovery efficiencies and was not competitive with the roasting process where large volumes of ore were handled. Because the precipitation process generally required a lower capital investment than a roasting operation, however, it found favor in locations where the supply of ore was insufficient to justify the capital expenditure required for a more efficient process.
In the precipitation process, the lead sulfide ore and metallic iron were charged to a reverberatory or blast furnace together with coke which was used as a fuel. Although the lead in the ore was directly reduced to the elemental state by reaction with the iron, the sulfur content of the ore was oxidized by air drawn into the furnace and sulfur dioxide was thus generated. The thermodynamics of the process were not well known nor were the conditions under which the sulfur might be retained in the condensed state.
Because of the theoretical potential for the precipitation process to proceed as follows: EQU PbS + Fe .fwdarw. Pb.degree. + FeS
this scheme raises the possibility of commercial recovery of lead from galena without significant SO.sub.2 generation. Prior to the present invention, however, the conditions under which the precipitation process could be operated, either for high lead recovery or SO.sub.2 suppression, were unknown to the art.